Electric double-layer capacitors and secondary batteries are known as conventional techniques for storing electrical energy (for example, see Patent Document 1). Electric double-layer capacitors are markedly superior to secondary batteries in terms of lifespan, safety and output density. However, electric double-layer capacitors suffer from low energy density (volumetric energy density) compared with secondary batteries.
Accordingly, in order to improve the energy density of electric double-layer capacitors, techniques have been proposed for increasing the electrostatic capacitance or the applied voltage of electric double-layer capacitors.
One known technique for increasing the electrostatic capacitance of an electric double-layer capacitor involves increasing the specific surface area of the activated carbon that constitutes the electrodes of electric double-layer capacitor. Currently known activated carbon has a specific surface area of 1,000 m2/g to 2,500 m2/g. In an electric double-layer capacitor in which this type of activated carbon is used as the electrodes, an organic electrolyte obtained by dissolving a quaternary ammonium salt in an organic solvent or an aqueous solution electrolyte of sulfuric acid or the like is used as the electrolyte.
With an organic electrolyte, because the voltage range that can be used is broad, the applied voltage can be increased, enabling the energy density to be increased.
With an aqueous solution electrolyte, the electrolysis reaction of water becomes rate-limiting, and increasing the applied voltage is difficult. However, in the case of an aqueous solution electrolyte, the adsorbed and desorbed ions are hydrogen ions and hydroxide ions, which are smaller than the cations and anions in the case of an organic electrolyte. Accordingly, with an aqueous solution electrolyte, the electrostatic capacitance can be increased compared with an organic electrolyte.
Further, redox capacitors that use ruthenium oxide or the like as an electrode material and utilize the pseudo-capacitance of the ruthenium oxide are also known. Redox capacitors offer the advantage of having a large electrostatic capacitance compared with electric double-layer capacitors that use activated carbon. However, redox capacitors have a problem in that, compared with electric double-layer capacitors using activated carbon or the like, the stability of the charge-discharge cycle is low, and another problem in that the electrode material such as ruthenium oxide is an expensive and rare resource, meaning industrial application is difficult.
Furthermore, pseudo electric double-layer capacitors that use conductive polymers are also known.
One known technique for increasing the applied voltage of an electric double-layer capacitor is a lithium ion capacitor that utilizes the principles of an electric double-layer capacitor. Lithium ion capacitors are also known as hybrid capacitors. In a lithium ion capacitor, one of the electrodes that constitutes an electric double-layer capacitor is replaced with graphite or hard carbon or the like that acts as the anode material of a lithium ion secondary battery, and lithium ions are inserted into the graphite or hard carbon. Lithium ion capacitors have the advantage that the applied voltage is larger than that for a typical electric double-layer capacitor, namely a capacitor in which both electrodes are composed of activated carbon. However, when graphite is used for an electrode, a problem arises in that propylene carbonate cannot be used as the electrolyte. When graphite is used for an electrode, propylene carbonate undergoes electrolysis, and decomposition products of the propylene carbonate adhere to the surface of the graphite, causing a deterioration in the lithium ion reversibility. Propylene carbonate is a solvent that can operate even at low temperatures. When propylene carbonate is used in an electric double-layer capacitor, the resulting electric double-layer capacitor can be used even at −40° C. Accordingly, in a lithium ion capacitor, a hard carbon that is unlikely to cause electrolysis of propylene carbonate is used as the electrode. However, compared with graphite, hard carbon has a low capacitance per unit volume of the electrode, and the voltage is also lower than that obtained with graphite (and adopts a noble potential). As a result, problems arise, including a lowering of the energy density of the lithium ion capacitor.